1. Field of the Invention
This invention relates to an improved structure of printed circuit board (PCB) under pushbutton, more particularly, to a PCB, wherein a circuit pattern can be printed on an insulation membrane and a base layer with waterproof function to fit various button groups, and to lessen thickness of a keyboard when being assembled with other key parts.
2. Description of the Prior Art
A pushbutton or combination keyboard in large or small scale can be found almost everywhere in household electrical appliances, test instruments, computers, etc. for easy control operation.
A membrane printed circuit board (PCB) used in a personal computer or notebook computer is usually composed of an upper and a lower layer membrane and an insulation layer inserted therein. As there is no glue applied among those three layers, hence, a contact point in the upper and the lower layer membrane respectively will contact each other when a rubber body of pushbutton is pressed down to squeeze air. However, in case a water drop happens to go into a pushbutton, no sooner has the rubber body restored from the previous state of being pressed then moisture is drawn into and intrudes upon the inside membrane PCB under a pushbutton which is likely to cause a short-circuit problem. Further, in assembling a keyboard, the mentioned PCB is commonly laid firstly on the base layer made of a metallic material before anchoring other button components that inevitably increases thickness, weight, and cost of the keyboard.
Another kind of PCB contains an upper and a lower flexible layer membrane, wherein a plurality of conductive portions is printed on the upper layer membrane while a plurality of contact portions corresponding with above conductive portions is printed on the lower layer membrane; and, an insulation portion is printed on each contact portion. The upper and the lower layer membrane are glued to form a membrane PCB of this kind.
Such a membrane PCB is also waterproof, nevertheless, it""s laid on a metallic base layer before anchoring other button components that inevitably increases thickness, weight, and cost of the keyboard as mentioned above.
The primary object of this invention is to eliminate abovesaid defects by printing a circuit pattern on an insulation membrane and a base layer directly to fit various button groups with waterproof function and reduce thickness of the keyboard and production cost after assembling with other button components.
Another object of this invention is to provide an insulation portion of a PCB circularly and radially to be disposed around a contact portion of an insulation membrane, or circularly disposed in geometrical shapes, such as a circle, square, etc.
A further object of this invention is to transfer an insulation portion on an insulation membrane to a base layer, so that, it requires printing a contact portion and a circuit pattern only when fabricating the insulation membrane.
To realize abovesaid objects, the PCB of this invention comprises a base layer and an insulation membrane, wherein a plurality of conductive portions and corresponding contact portions are printed on the base layer and the insulation membrane respectively; a plurality of insulation portions is printed on each contact portion along its circumference; and, no glue will be applied on a corridor between every pair of the conductive portions in the base layer and the contact portions in the insulation membrane when combining the insulation membrane to the base layer by gluing or direct printing. By this arrangement, an air channel can be reserved for being applicable to various button groups or waterproof membrane PCBs.